A band-pass microstrip filter is generally made up by placing a plurality of microstrip resonators each resonating at the center frequency of a pass band at intervals corresponding to a coupling coefficient to thereby realize frequency characteristics satisfying design requirements. Therefore, when manufacturing the band-pass microstrip filter, the microstrip resonators have to be adjusted so as to resonate at the center frequency of the pass band even if the capacitance between the ground surface and the microstrip pattern has changed due to variations in the thickness of the substrate or the like. Such adjustment is carried out after the microstrip filter is manufactured and packaged using adjust screws made of metal or dielectric material while measuring the frequency characteristics of the filter. Alternatively, in especial when the center frequency of the resonators is low, it is necessary to perform an adjustment process for adjusting the center frequency to a design value by shortening the length of the resonators by trimming, etching, etc. after the measurement of the frequency characteristics. Although the latter adjustment process requires extremely complicated steps as compared with the former, the center frequency can be adjusted more precisely than if the adjust screws were employed. Besides, the latter adjustment process, free from variation or change over time caused by the loosening of screws etc. in principle, has the advantage of high reliability. For example, in narrow-band-pass high-temperature superconducting microstrip filters with a relative bandwidth of about 1% of 2 GHz, which are employed in mobile communications, when MgO substrates 0.5 mm thick are used, there generally are variations of ±8 μm or ±1.6% in thickness from substrate to substrate, and thereby the capacitance between strip patterns and ground surfaces also varies. Consequently, the resonance frequency of the resonators deviates from the design value, and the center frequency of the filter also deviates from the design value. Therefore, the processes for adjusting the deviated center frequency to the design value have been required. In other words, after producing the filter pattern, it has been necessary to package the filter, measure the frequency characteristics of the filter, return the filter to the manufacturing process depending on the measurement result, and shorten the length of the resonators by a proper length by means of trimming or etching.